title: Water-use efficiency and relative growth rate mediate competitive interactions in Sonoran Desert winter annual plants.
abstract: OBJECTIVE
  A functional approach to investigating competitive interactions can provide a mechanistic understanding of processes driving population dynamics, community assembly, and the maintenance of biodiversity. In Sonoran Desert annual plants, a trade-off between relative growth rate (RGR) and water-use efficiency (WUE) contributes to species differences in population dynamics that promote long-term coexistence. Traits underlying this trade-off explain variation in demographic responses to precipitation as well as life history and phenological patterns. Here, we ask how these traits mediate competitive interactions. •
  
  
  METHODS
  We conducted competition trials for three species occupying different positions along the RGR-WUE trade-off axis and compared the effects of competition at high and low soil moisture. We compared competitive effect (ability to suppress neighbors) and competitive response (ability to withstand competition from neighbors) among species. •
  
  
  RESULTS
  The RGR-WUE trade-off predicted shifts in competitive responses at different soil moistures. The high-RGR species was more resistant to competition in high water conditions, while the opposite was true for the high-WUE species. The intermediate RGR species tended to have the strongest impact on all neighbors, so competitive effects did not scale directly with differences in RGR and WUE among competitors. •
  
  
  CONCLUSIONS
  Our results reveal mechanisms underlying long-term variation in fitness: high-RGR species perform better in years with large, frequent rain events and can better withstand competition under wetter conditions. The opposite is true for high-WUE species. Such resource-dependent responses strongly influence community dynamics and can promote coexistence in variable environments.
authors.affiliation: Department of Ecology and Evolutionary Biology, P. O. 210088, University of Arizona, Tucson, Arizona 85721 USA.
authors.forename: Jennifer R
authors.initials: JR
authors.lastname: Gremer
authors.forename: Sarah
authors.initials: S
authors.lastname: Kimball
authors.forename: Katie R
authors.initials: KR
authors.lastname: Keck
authors.forename: Travis E
authors.initials: TE
authors.lastname: Huxman
authors.forename: Amy L
authors.initials: AL
authors.lastname: Angert
authors.forename: D Lawrence
authors.initials: DL
authors.lastname: Venable
mesh_terms: D001130:Arizona; D018533:Biomass; D003889:Desert Climate; D019684:Magnoliopsida / Q000502:physiology*; D063245:Plant Development / Q000502:physiology*; D012621:Seasons; D012639:Seeds / Q000254:growth & development; D014867:Water / Q000502:physiology*
chemical_list: D014867:Water
keywords: Erodium texanum; Plantago insularis; Stylocline micropoides; coexistence; competition; condition dependence; ecophysiology; functional traits; trade-offs